This invention relates to an infrared gas analyzing apparatus of the type wherein first and second gas-filled detecting chambers arranged in series are commonly disposed in the path of light measurement rays and light comparison rays, and a gas sample introduced in a vessel disposed in the light measuring path is analyzed by measuring the difference between the light energies absorbed by the first and second detecting chambers, respectively, and wherein the effects of an interference component in the sample, such as water vapor, are compensated for. The invention is also applicable to implement zero adjustment using a reference gas circulated through a vessel.
An example of a prior art infrared ray gas analyzer, as disclosed in U.S. Pat. No. 3,162,761, is shown in FIG. 1, where L designates a source of infrared rays divided by a chopper CH into reference light rays Iv and measuring light rays Is. The chopper CH is rotated by a motor M and has openings O1 and O2, as shown in FIG. 2, wherein opening O1 passes the measuring light rays Is and opening O2 passes the reference light rays Iv.
The measuring and reference light rays are formed by the chopper CH in a periodically alternating manner, the measuring light rays being directed through a measuring vessel S and the reference light rays being directed through a reference vessel V. The vessels S and V are formed in an integral body having light transparent windows R1 and R2 on each end. A sample containing the gas component to be analyzed is introduced into the measuring vessel S through inlet and outlet tubes Z1, Z2, as shown by the arrows, and a gas having no absorbing characteristic for infrared rays, such as nitrogen, is sealed in the reference vessel V. Accordingly, the infrared measuring light rays Is that pass through the vessel S are subjected to absorption depending on the density of the gas component to be analyzed, while the infrared reference light rays Iv that pass through the vessel V are not subjected to any absorption. After passing through the measuring and reference vessels, respectively, the alternate light rays Is and Iv are directed into a detector D.
The detector D has a first detecting chamber D1 and a second detecting chamber D2 arranged in series with respect to the paths of the infrared rays, and both of these chambers are filled with a gas of the same kind as the component gas to be analyzed. R3 and R4 designate light transparent windows, and K designates a condenser microphone type of detector connected between the chambers D1 and D2. The difference in pressure variation in the two chambers based on the difference in their infrared ray absorption of the measuring light rays Is is sensed by the condenser microphone K as a capacity variation, and this is appropriately converted into a corresponding voltage variation which indicates a value of the component gas being analyzed.
FIG. 3 shows the energy absorbing characteristics of the measuring light rays Is in the chambers D1 and D2. The area A1 defined by and within curve a indicates the magnitude of the measuring light ray energy absorbed in chamber D1, and the area A2 defined by and between the curves a and b indicates the magnitude of the measuring light ray energy absorbed in chamber D2. The difference .DELTA.A between the areas A1 and A2 varies in proportion to the density of the component gas being analyzed contained in the sample gas, and an electrical signal corresponding to .DELTA.A is generated by the electrical circuit including the condenser microphone. The difference .DELTA.A is always constant when the density of the gas component being analyzed is constant.
In many cases, however, a gas having an infrared-ray absorbing range identical with or partially overlapping that of the gas component being analyzed coexists in the sample gas, and due to this interference component an error is introduced into the analysis results. In FIG. 3 the curve c represents the absorption of infrared rays due to such an interference component. The absorption of infrared rays in the detector D by the interference component affects both of the magnitude of the light energy (area A1) absorbed in chamber D1 and the magnitude of the light energy (area A2) absorbed in chamber D2. When the ratio between the areas A1 and A2 is set such that the area abc is equal to the adjacent area bcde, the difference .DELTA.A between areas A1 and A2 remains constant regardless of the absorption of infrared rays by the interference component, thus eliminating the harmful effects thereof. To suitably select the ratio between the absorption characteristic of the chambers D1 and D2 a procedure is used which includes determining the density of the gas filling the chambers, and the shape and volume of the respective chambers. According to this procedure, however, precise determinations are extremely difficult and the realization of all of the factors and conditions involved is quite complex.
A further prior art infrared ray gas analyzer is disclosed in U.S. Pat. No. 2,951,939, wherein a single measuring vessel S is employed and a movable light shielding plate is disposed between the detecting chambers D1 and D2 to implement zero adjustment. That is, with a reference gas in the measuring vessel and the chambers dimensioned so that A2&gt;A1, the shielding plate is adjusted to cut off or block a portion of the light energy transmitted through chamber D1 before it enters chamber D2, whereby A2 is reduced until it equals A1. This requires the detecting chambers D1, D2 to be constructed as two separate bodies, however, which is relatively difficult and costly in view of the degree of precision required, and the accurate optical alignment of the separate chambers presents a further problem, as does the mechanical stability of the light shielding plate.